Lawn Mower Blade Sharpening

It’s easy when you know what you’re doing!

Your lawn mower blade is dull. Sharpen the blade twice each season to help maintain a green, healthy lawn. A sharp blade not only cuts blades clean so grass plants recover quickly, it helps reduce your lawn mowing time.

By the DIY experts of The Family Handyman Magazine:July / August 2006

Overview

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Photo 1: Remove spark plug

Pull the spark plug wire from the spark
plug to prevent the motor from accidentally
starting. Tape or tie it back so it doesn't flop
back into contact with the plug.

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Photo 2: Tip mower on its side

Turn the mower onto its side with the air filter and carburetor side up. This keeps oil and gas from dripping into the air filter.

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Photo 3: Remove the blade

Wedge a short 2x4 between the blade and the deck to clamp the blade. Loosen the bolt (or nut) with a long-handled wrench. Turn counterclockwise. Remove the bolt and blade.

One of the best ways to encourage a greener, fuller
and healthier lawn is to sharpen your lawn mower blade.

A dull blade rips and pulls the grass blades, leaving ragged tears that both weaken the plant and promote fungal growth and other grass diseases. A sharp blade, on the other hand, cuts cleanly, allowing the plant to heal and recover quickly. Sharp blades also let you complete your lawn-cutting chore faster and with less stress on the mower.

Sharpening is a simple task, even for a novice. It’ll take a few sharpenings to master the technique. After that, the chore will take less than 10 minutes. Plan to do it twice every mowing season. We show here the steps that will work for just about any walk- behind mower. Riding mowers require different blade removal techniques, which we won’t show here.

Play it safe when removing the blade

We recommend always removing the spark plug when you're working on the blade. If the piston happens to be at the top of the compression stroke, a little bump to the blade might force the piston over the “hump” and into the power stroke. If that happens, the blade will lurch around and could just break your hand!

Then look for the carburetor and air filter. The carburetor is usually easy to recognize because it has throttle cables running to it. If you keep this side up when you tip your mower over to get at the blade (Photo 2), you won’t get a smoke cloud from leaking oil the next time you start it. Some mowers have gas caps with air holes that could leak a little gas onto your garage floor, so work outside or keep a rag handy to clean up drips. Once the blade is off, set the mower back onto all four wheels until you’re ready to reinstall your blade.

You’ll usually find a single bolt or nut holding the blade on. It’s usually very tight and you’ll need to clamp the blade to loosen it. The 2x4 method we show (Photo 3) is simple, quick and safe. Don’t use your foot! A good tool to keep handy to loosen the bolt is a breaker bar or long-handle wrench with a socket to match the bolt. It’ll give you plenty of leverage to loosen extremely tight bolts, and you can keep your knuckles well away from the blade when bearing down. Use a squirt of penetrating oil on really rusted, stuck bolts. Wait 10 minutes to give it time to work.

Spray paint the blade before removal.

Mark Your New Blade

Mark your blade with spray paint before you remove it so you know which way to reinstall it. Mower repair pros say that the biggest mistake homeowners make is installing a blade upside down after sharpening it. The blade won’t cut—and they’ll go nuts trying to figure out why!

Tip: Keep a second blade on hand. The store will probably be closed when you need it!

Sharpen it with a file

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Photo 4: Sharpen blade with file

Clamp the blade in a vise and sharpen the cutting edge with a mill bastard file, held at the same cutting angle as before. File until the blade is “butter knife” sharp.

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Proper filing angle

Original cutting angle

Once you remove the blade, examine it to determine whether to sharpen it or replace it. We recommend that you sharpen it with a hand file (Photo 4). Mower blades are made from fairly soft steel. You can sharpen most with fewer than 50 strokes of a clean, sharp “mill bastard” file that’s at least 10 in. long. Grinders also work, and much more quickly. (Pros use them.) But they’re more difficult to control and you might overheat and ruin the blade.

Always sharpen from the top side of the cutting edge; this will give you the longest-lasting edge on the blade. The file cuts in one direction only, on the push stroke; you’ll feel it bite into the steel on the blade. If you don’t feel that cutting action, your file is probably dull or you’re not pressing down hard enough. Don’t try to make your blade razor sharp; it’ll dull more quickly. “Butter knife” sharp will do.

Sharpening mulching blades is sometimes more difficult. Mulching blades may have longer or curved cutting edges, and you may need several types of files to sharpen them. In some cases, you may have to resort to a 4-1/2-in. angle grinder. If your blade is too difficult to sharpen, take it to a hardware store or a blade sharpening service.

Balance it before reinstalling

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Photo 5: Check blade balance

Hang the blade on a nail to check the balance. If one side dips, file a bit more off that side until the blade remains horizontal.

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Photo 6: Reinstall the blade

Reinstall the blade and screw in the bolt. Then wedge the 2x4 back in and tighten the bolt firmly with your socket and breaker bar.

Before you reinstall the blade, be sure to balance it. An unbalanced blade will cause vibration and possibly ruin the blade shaft or bearings. To check the balance, simply drive a nail into a stud and set the blade onto it like an airplane propeller (Photo 5). If one side falls, it’s heavier, and you have to file more metal off it. Keep filing until the blade stays level.

Reinstall the blade and hand-tighten the bolt. Insert the 2x4 in the reverse direction so you can bear
down on the breaker bar to tighten the bolt. It’s difficult to overtighten the bolt. Mower sharpening pros say that the second most common mistake they see is undertightening the bolt. A loose blade throws off the engine timing and sometimes makes the
mower hard to start.

Keep second blade and tools handy.

No Excuses

To get in the habit of keeping your blade sharp, dedicate a set of tools for sharpening only. Hang them nearby so they’re ready to go. And keep a second, sharp blade handy too. You can slip it on and sharpen the dull one later.

Only use the blades recommended for your mower.

Buying a New Blade

Always replace your blade with an exact replacement blade, or the blade recommended in your owner’s manual. Resist the temptation to convert your regular straight-blade mower to a fancier mulching mower by simply changing the blade. Your mower probably won’t work any differently than before, and it may not work as well. The mower deck on a straight- blade mower is shallow and has a side discharge to eject the grass clippings quickly. A mulching mower has a deeper deck without a side discharge; the grass is chopped three or four times before it drops to the ground. The mower design is as important as the blade.

A thin trailing edge: The trailing edge, or fin, is the edge opposite the cutting edge. This fin is often slanted upward, which creates an updraft to lift the grass and grass clippings. Dust and sand will wear this fin down. When it’s thin, replace the blade.

A bent blade: Set your old blade on your work-bench and check for bends. If you'e unsure, compare it with a new blade.

Dents in the cutting edge: Replace blades that have deep dents that you can't file out and erosion from wear and sharpening. Also replace any blade that has cracked.

Do You Need a New Blade?

Examine your blade when you remove it and look for the problems shown. If you’re unsure of the condition of the blade, take it to a hardware store or home center and compare it with a new one.

Tools & Materials List

Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

Socket/ratchet set

File

Rags

Vise

spark plug socket

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.